Apparently the issue is with the reduced amount of oxygen in the air at higher altitudes
which doesn't allow the oxygen sensors in the units to function as designed.

See: http://www.ventfree.org/content/view/43/18/

it looks like it actually means the safety off sometimes kicks in more at higher altitudes but that the device itself is not unsafe. cool, and the page also says that 4500 mark was kind of arbitrary and that they seem to work ok on up to 6000 feet.

plus this house is NOT well sealed or insulated so I have that going for me too!

The reason there's an altitude limitation is that there's no oxygen up here and it's likely to start producing a lot of carbon monoxide. I also wouldn't use it in a super insulated house. Note it can't be sold in California or Canada, it won't meet their strict indoor emissions controls.

Since you're right at the limit, you might get away with it, but get a carbon monoxide detector.

I'd be more inclined to get a unit that works with a through the wall vent.

looks like the "problem" is the O2 sensor shutting the gas off rather than any kind of real problem, so I went ahead and ordered it, will let everybody know how it works in a few months. I think you are closer to 6000 am I right? do they sell them up there? they have them in the hardware stores and at tractor supply in town but that is at 3500 so I wasn't positive they would work up here. from the link I am willing to give one a try.

Everything up here has to be vented, pretty much. I have a woodstove and didn't seal up any of the cracks when I moved in.

I think they've probably been conservative in their altitude limitation. I would still get the CO detector, they're really cheap insurance for an unvented gas unit.

I have one because my floor furnace was installed in 1946. It's older than I am.

so, I have had the thing installed and running for 2 months now

it works so freaking good! we have not built a single fire in the fireplace yet this winter. this thing cranks and I never have it above the "3" setting! my gas bill has doubled but it is still a hell of a lot cheaper than running useless electric heaters. I am so impressed.

no problem with it shutting down (of course this house is so drafty there is always fresh air) or any other problems, no odor no smoke, nothing. well wait - unlike central heating this releases a bit of moisture so on cold days the windows fog up a little. the house plants and my skin are liking that little side effect!

I have a real heat gradient in the living room now and need to install a ceiling fan or two, but that will have to wait until next year, I think. too many other priorities. for now I have a box fan aimed at the ceiling and just run it on low for a minute or two whenever I think about it to stir the air around.